An MRI apparatus measures magnetic resonance (hereinafter, referred to as NMR) signal of nucleus spins of an object disposed in an imaging space inside a homogeneous static magnetic field, and displays a nucleus spin density distribution, a relaxation time distribution, or the like of the object as a tomographic image. The MRI apparatus includes a static magnetic field generation device which generates a static magnetic field, and a gradient magnetic field generation device which generates a gradient magnetic field for adding position information to an NMR signal.
If disturbance occurs in the homogeneity of a static magnetic field generated by the static magnetic field generation device, the linearity of a gradient magnetic field superposed thereon deteriorates. Therefore, deviation occurs in position information added to an NMR signal, and this causes strain or a loss of an image. The strain or the loss of an image damages the accuracy or sharpness of the image and thus seriously impedes diagnosis. Therefore, extremely high homogeneity is required for a static magnetic field of an imaging space.
On the other hand, since the intensity of an NMR signal is substantially proportional to the intensity of a static magnetic field, a static magnetic field generation device which generates a static magnetic field with large intensity is desirable in order to obtain high quality MRI image. As mentioned above, high homogeneity and large magnetic field intensity (high magnetic field) are required for a static magnetic field generated by the MRI apparatus, and thus the static magnetic field generation device is one of considerably principal constituent elements of the MRI apparatus.
It is known that a static magnetic field generation device using a superconducting magnet stably forms a static magnetic field with high homogeneity and large intensity in an imaging space for a long period of time. It is known that a cylindrical superconducting magnet has a shape for generating a high magnetic field with high efficiency. The cylindrical superconducting magnet has a structure in which a plurality of superconducting coils are disposed in a cryostat, or a low-temperature container in which liquid helium or other low-temperature freezing media are enclosed.
The cryostat or the low-temperature container is disposed in a vacuum vessel, and a radiation shield for blocking permeation of heat from the outside is disposed inside the vacuum vessel. A freezer is attached to the vacuum vessel, and a cooling portion of the freezer is connected to the cryostat or the low-temperature container and the radiation shield so as to maintain a low temperature.
In an MRI apparatus in which a superconducting magnet is used for a static magnetic field generation device, a volume and a shape of an imaging space known as a field of view (FOV) differs depending on a necessary imaging target, but are defined by a peak-to-peak value of the magnetic field homogeneity, and the space has substantially a spherical shape. Recently, in an MRI apparatus in which the central magnetic field intensity is 1.5 teslas, a peak-to-peak value of the homogeneity of a static magnetic field has been generally several tens of ppm (about 20 to 40 ppm) at an FOV with a diameter of about 45 to 50 cm.
The homogeneity of a static magnetic field in a superconducting magnet is mainly defined by arrangement of superconducting coils, and thus the arrangement is designed so that a necessary homogeneous magnetic field is generated in a desired space. However, actually, it is hard to realize magnetic field homogeneity as designed due to a manufacturing dimension error of a superconducting magnet, and the homogeneity of a static magnetic field in a single superconducting magnet is about several hundreds of ppm at an FOV with a diameter of about 45 to 50 cm. Thus, in order to correct non-homogeneity of a static magnetic field in the superconducting magnet, a method is generally used in which minute magnetic pieces called a passive shim are disposed around an imaging space, and the static magnetic field is finely adjusted.
PTL 1 proposes a method in which an amount and positions of magnetic pieces to be disposed to adjust a magnetic field in a static magnetic field generation device are obtained through computation. In the adjustment method disclosed in PTL 1, a spatial distribution of a magnetic field in an imaging space is measured, and an amount and arrangement of magnetic pieces for correcting an error magnetic field with respect to a desired homogeneous magnetic field are calculated by using an eigen distribution function obtained through singular value decomposition.